Behind the brake pedal is a switch. Either the switch is faulty or it has worked loose. You remove it by twisting it, pulling off the connector and then replacing it with another one. I had the problem when my foot slipped off the pedal and it went back and dislodged the switch.
as answered in motoring the switch is located beneath the glovebox on these cars as they were originaly manufactured for left hand drive in france, there is a bar that runs along the back of the dash, the switch is located on this bar beneath the glovebox
same problem with my wifes car 9 times out of 10 the switch has come loose it is below glove box behind the bar try puting it back in get someone to hold brake pedal down so you have easier acses to switch you push it back in and twist to lock it back in place releese brake and check lights if no joy an after market replacement is cheaper than a dealer part and can be picked up from any car spares shop for no more than 15 quid